OTTAWA - Liberals are poised to derail Prime Minister Stephen Harper's cherished Senate reform agenda.

Sources told The Canadian Press that Liberal members of the Senate's legal and constitutional affairs committee will recommend Wednesday shelving a bill that would impose eight-year limits on senatorial terms.

Liberals, who hold a big majority in the upper house, will insist that the Senate not proceed with the bill until the Supreme Court of Canada determines whether it's constitutional for Parliament to proceed unilaterally, without provincial consent.

The Liberal move follows submissions from four provinces -- Ontario, Quebec, New Brunswick and Newfoundland and Labrador -- insisting that provincial consent is needed to reform the Senate.

The four have demanded a halt to both the term-limit bill and another bill, currently before the House of Commons, that would create a process for electing senators.

To break the impasse, Harper's government would have to agree to refer the matter to the top court.

Even if that happened and the court gave the green light to resuscitate the bill, the Liberals are proposing amendments so that senators would be limited to serving one non-renewable 15-year term and would still have to retire at age 75.

Given the depth of provincial opposition, Liberal Leader Stephane Dion said Wednesday that Harper needs to rethink Senate reform.

"I think the prime minister should start from scratch,'' Dion said.

"He started a process about the Senate that was politically driven again, about his own agenda, and the constitutional partners of our federation are saying, 'Hey, what are you doing? We need to look at that together. What is this unilateral way to proceed?'"

The Tories recently launched a fourth series of TV attack ads against Dion, portraying him as a weak leader who's unable to persuade Liberal senators to endorse the term-limit bill. Dion initially endorsed the idea in principle but wanted longer, non-renewable terms of up to 15 years.

Dion said it's ironic that the Tories' are attacking him on Senate reform when the problem clearly lies with a prime minister who is dismissive of the provinces.

"Federalist practice is that you work with your constitutional partners on these issues and the prime minister didn't."

He noted that Harper, after almost 18 months in office, still hasn't bothered to hold a formal first ministers meeting.

"Is it open federalism? Come on,'' Dion scoffed.